Suitable fixing systems are usually used for the locking of pieces to be machined on a machine tool.
In this regard it is known that, in order to stably lock a piece, it is sufficient to fix it at three predetermined gripping points which enable the operator to univocally fix the piece zero point with respect to the machine tool zero point.
Depending on the type of machining operation to be performed on the machine tool, however, it sometimes occurs that three gripping points only are not enough to completely stabilize the piece on the machine tool.
This need is particularly felt, for example, in the automotive sector, in which the continuing need to optimize production cycles has led to the use of tools that operate at very high speeds, discharging very strong forces and vibrations on the piece.
Still in the automotive sector, moreover, the use is increasingly more frequent of particularly lightweight materials such as aluminum which, nevertheless, during working on the machine tool, are not able to ensure the same resistance as materials such as cast iron and steel.
It does, therefore, occur that the machining of some portions of the pieces, particularly if they are very distant from the gripping points, can result in the deformation of the pieces themselves by the tool, something which negatively affects the quality of the machining operation with the risk of obtaining out-of-tolerance pieces.
To overcome these drawbacks, the use is known of additional fixing systems which define extra gripping points over and above the three predetermined gripping points.
Such additional fixing systems, which are positioned after positioning the piece on the machine tool and locking it at the three predetermined gripping points, do have a number of drawbacks related to the great difficulty of locking the piece without these deforming it.
The locking of the piece at four or more points, in fact, defines a hyperstatic structure wherein the number of degrees of constraint of the piece is greater than the number of its degrees of freedom.
In such hyperstatic structure the presence of a higher number of constraints can generate states of internal tension whenever the fixing systems are not fitted at the exact points of the piece.
While it is hypothetically possible to place a fixing system with extreme precision in a predetermined position of the machine tool, nevertheless it is virtually impossible to predict at what exact point of the piece the fixing system will come into contact due to the fact that the piece (e.g., obtained by casting or molding) still has rough surfaces to be machined.
The result, therefore, is that the piece is fixed in four or more points to increase its stability during machining but is however subjected to states of internal tension which may determine its deformation during clamping.